The present invention relates to the evaluation of metallic alloys for resistance to stress corrosion cracking. In particular, the present invention is concerned with the evaluation of NiCrFe alloys for resistance to stress corrosion cracking in deaerated primary water (i.e. low oxygen content water used in nuclear reactors).
The problem of stress corrosion cracking of metallic alloys, particularly NiCrFe alloys, during use in deaerated primary water has existed for quite some time. Evaluation procedures for testing these alloys to determine their suitability for use in this particular environment have been developed. These known evaluation procedures are based upon the observation that alloys which do possess adequate resistance to stress corrosion cracking in deaerated primary water have carbides precipitated primarily in their grain boundaries. These carbide deposits are formed during the actual production of the alloys.
In previous work the evaluation of the NiCrFe alloys involved complicated metallographic procedures. A typical procedure comprises grinding and polishing the alloys followed by etching the alloy with nital. The nital etch is used to determine the grain boundary locations. The alloy is marked to identify the microscopic area and a photomicrograph of the area at high magnification (e.g. 500x) is taken. Subsequently, the alloy is reground and repolished to eliminate the effects of the nital etchant. The refinished alloy is re-etched with phosphoric acid to determine the location of the carbide precipitates. A photomicrograph at 500x establishes these locations. Finally, the two photomicrographs are compared to determine the continuity and location of the carbide precipitate enabling the investigator to predict the resistance of the tested alloy to stress corrosion cracking in deaerated primary water. The problems with this procedure are evident. It is not only long and difficult, but evaluation is limited to extremely minute areas (i.e. 8.times.10.sup.-5 /in.sup.2 for a 4.times.5 inch photomicrograph). Accordingly, it is often necessary to examine several different locations to ensure proper evaluation. This, of course, necessitates duplicating the above described procedure, increasing the time required for complete evaluation.
A number of unsuccessful attempts have been made to improve the above described procedure. For example, attempts have been made to use a single etch which attacks the carbides. However, this procedure left doubt as to the location of the grain boundaries. Conversely, if the grain boundaries are delineated by a single etch, the location of the carbide precipitate becomes a problem. Accordingly, development of a relatively uncomplicated and accurate evaluation procedure has remained a problem because of the necessity of including two etching techniques.